The aggregation and deposition kinetics of two multiwalled carbon nanotubes (MWNTs) with different degrees of surface oxidation are investigated using time-resolved dynamic light scattering (DLS) and quartz crystal microbalance with dissipation monitoring (QCM-D), respectively. Carboxyl groups are determined to be the predominant oxygen-containing surface functional groups for both MWNTs through X-ray photoelectron spectroscopy (XPS). The aggregation and deposition behavior of both MWNTs is in qualitative agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The critical coagulation concentration (CCC) of the highly oxidized MWNTs (HO-MWNTs) is significantly higher than the lowly oxidized MWNTs (LO-MWNTs) in the presence of NaCl (210 and 53 mM, respectively) since HO-MWNTs have a higher surface charge density. In contrast, the aggregation inverse stability profiles of HO-MWNTs and LO-MWNTs are identical and yield comparable CCCs (0.9 and 1.0 mM, respectively) in the presence of CaCl(2). Similar to the results obtained from the aggregation study, HO-MWNTs are considerably more stable to deposition on silica surfaces compared to LO-MWNTs in the presence of NaCl. However, both MWNTs have the same propensity to undergo deposition in the presence of CaCl(2). The remarkable similarity in the aggregation and deposition kinetics of HO-MWNTs and LO-MWNTs in CaCl(2) may be due to Ca(2+) cations having a higher affinity to form complexes with adjacent carboxyl groups on HO-MWNTs than with isolated carboxyl groups on LO-MWNTs.

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The influence of solution chemistry on the kinetics and reversibility of the deposition of multiwalled carbon nanotubes (MWNTs) on model biological membranes was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). Supported lipid bilayers (SLBs) comprised of zwitterionic 1,2-dioleoyl-sn-glyero-3-phosphocholine (DOPC), as well as DOPC vesicles, were used as model cell membranes. Under neutral pH conditions, the deposition kinetics of MWNTs on SLBs increased with increasing electrolyte (NaCl and CaCl2) concentrations. In the presence of NaCl, favorable deposition was not achieved even at a concentration of 1 M, which is attributed to the presence of strong repulsive hydration forces due to the highly hydrophilic headgroups of SLBs. Conversely, favorable deposition was observed at CaCl2 concentrations above 0.5 mM when the charge of SLBs was reversed from negative to positive through the binding of Ca(2+) cations to the exposed phosphate headgroups. Favorable nanotube deposition was also observed at pH 2, at which the DOPC SLBs exhibited positive surface charge, since the isoelectric point of DOPC is ca. 4. When MWNTs on SLBs were rinsed with low ionic strength solutions at pH 7.3, only ca. 20% of deposited nanotubes were released, indicating that nanotube deposition was mostly irreversible. The deposition of MWNTs on DOPC vesicles under favorable deposition conditions did not result in any detectable leakage of solution from the vesicles, indicating that MWNTs did not severely disrupt the DOPC bilayers upon attachment.

Heteroaggregation with indigenous particles is critical to the environmental mobility of engineered nanomaterials (ENM). We studied heteroaggregation of ceria nanoparticles (n-CeO2), as a model for metal oxide ENM, with nanoparticles of pyrogenic carbonaceous material (n-PCM) derived from pecan shell biochar, a model for natural chars and human-made chars used in soil remediation and agriculture. The TEM and STEM images of n-PCM identify both hard and soft particles, both C-rich and C,O,Ca-containing particles (with CaCO3 crystals), both amorphous and "onion-skin" C-rich particles, and traces of nanotubes. Heteroaggregation was evaluated at constant n-CeO2, variable n-PCM concentration by monitoring hydrodynamic diameter by dynamic light scattering and ζ-potential under conditions where n-PCM is "invisible". At pH 5.3, where n-CeO2 and n-PCM are positively and negatively charged, respectively, and each stable to homoaggregation, heteroaggregation is favorable and occurs by a charge neutralization-charge reversal mechanism (CNCR): in this mechanism, primary heteroaggregates that form in the initial stage are stable at low or high n-PCM concentration due to electrostatic repulsion, but unstable at intermediate n-PCM concentration, leading to secondary heteroaggregation. The greatest instability coincides with full charge neutralization. At pH 7.1, where n-CeO2 is neutral and unstable alone, and n-PCM is negative and stable alone, heteroaggregation occurs by a charge-accumulation, core-shell stabilization (CACS) mechanism: n-PCM binds to and forms a negatively charged shell on the neutral surface of the nascent n-CeO2 core, stabilizing the core-shell heteraggregate at a size that decreases with n-PCM concentration. The CNCR and CACS mechanisms give fundamental insight into heteroaggregation between oppositely charged, and between neutral and charged nanoparticles.

The release of multiwalled carbon nanotubes (MWNTs) that were deposited on silica surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). MWNTs were deposited on silica surfaces at elevated NaCl and CaCl2 concentrations before being rinsed with eluents of different solution chemistries to induce their remobilization. Energetically speaking, the MWNTs were released from the primary energy minimum when the background NaCl or CaCl2 concentrations were decreased at pH 7.1. The increase in electrostatic repulsion between MWNTs and silica likely caused a reduction in the energy barrier, which enabled the release of MWNTs. The degree of release increased in a stepwise fashion when the nanotubes were sequentially exposed to eluents of decreasing electrolyte concentrations, possibly due to the heterogeneity in nanotube surface charge densities. The degree of release via a successive reduction in NaCl concentration was lower at pH 4.0 than at 7.1 due to MWNTs and silica surfaces exhibiting a less negative surface charge at pH 4.0. Most of the deposited MWNTs were released when the pH was decreased from 7.1 to 4.0 at 1.5 mM CaCl2. This was attributed to the elimination of calcium bridging between the carboxyl groups on MWNTs and silanol groups on silica surfaces.